The present invention relates to an image display apparatus such as a liquid display device and a backlight apparatus used therefor. More particularly, the present invention relates to an image display apparatus configured to reduce a luminance non-uniformity of a so-called tandem-type backlight apparatus which is comprised of a plurality of light guide plates.
In the recent years, it is required to further reduce a thickness (i.e. a depth size) of an image display apparatus such as a liquid display device. A so-called surface light source may be used as a backlight apparatus preferable for reducing a thickness of an image display apparatus, and the surface light source is such that a linear light source or a point-like light source is converted into a surface light source by a plate-like light guide plate composed of a transparent material such as acrylic or the like. For the surface light source device there is known a tandem-type surface light source in which a plurality of light guide plates are aligned each of which guides surface light converted from light of the light source to a liquid crystal panel side. The tandem-type surface light source is formed of a plurality of light guide plates in abutting or jointing, so that a shadow, dark line or bright line occurs at the boundary between adjacent light guide plates. That is, there occurs a step of luminance between a boundary portion of the light guide plate and a portion other than the boundary portion, resulting in the cause of luminance non-uniformity on a display screen.
In order to reduce the shadow, dark line or bright line at such a boundary between adjacent light guide plates, for example, a patent document JP-A-11-288611 discloses that in a surface light source device of tandem-type in which arranged are a plurality of sets each being formed of a primary light source and light guide blocks which guide surface light converted from light of the primary light source to a liquid crystal panel side, a tip of a certain guide block is superposed on a light guide block adjacent thereto to avoid the dark line or the bright line described above.
JP-A-2004-265635 discloses that a backlight apparatus is comprised of backlight units formed of a plurality of sets, each set including a light source, a light guide plate and a reflection unit, and a transparent acrylic plate intervenes between a backlight including a guide plate, a diffuser plate and a liquid crystal panel.
However, in the backlight apparatus described in JP-A-11-288611 and JP-A-2004-265635, there may be the case where the shadow, dark line or bright line at the boundary between light guide plates can not sufficiently be reduced.
For example, when the blocks or the backlight units are aligned, there may occur that light leakage due to positional displacement of the light guide plates at the boundary of blocks or backlight units causes a bright line. Reversely, shortage of light may cause a dark line in such a structure. Furthermore, in the prior art, since the luminance is made uniform within the block, there is a possibility that the bright line at the boundary may be conspicuous rather than otherwise.
As mentioned above, the light guide plate is comprised of resin such as acrylic or the like, so that it will cause thermal expansion due to heat from a light source such as LED (light emitting diode). If the light guide plates cause thermal expansion, they interfere with each other to press each other, so that the light guide plates may suffer deformation or breakage. To avoid the possibility, there may be a structure that the light guide plates are spaced from each other with a gap of, for example, as many as 0.1 mm to a few mm to absorb the thermal expansion in light guide plates. When such a gap is provided, the shadow, dark line or bright line at the boundary between light guide plates will be much conspicuous.
In such prior art, as disclosed in JP-A-11-288611 or JP-A-2004-265635, only by superposing the light guide plates on each other or making an acrylic plate to intervene between the diffuser plate and the light guide plate, there is a possibility that the luminance non-uniformity due to dark line or bright line at the boundary between the light guide plates cannot sufficiently be reduced.
In view of the above-mentioned problem, the present invention has been made, and provides a technology which can well reduce the luminance non-uniformity caused due to a shadow, a dark line or a bright line caused at the boundary between light guide plates where a backlight unit is formed by aligning a plurality of light guide plates.